I. Field of the Invention
This invention relates to a method for drying water-absorbent compositions of matter to further improve their particle agglomeration, "wet-out" and consequent absorbent properties. More particularly, the invention relates to a method for drying water-swellable, water insoluble ionic complexes of a water soluble anionic polyelectrolyte and polyvalent metal cations having a valence of at least three while retaining the granular form thereof.
II. Brief Description of the Prior Art
Recently there has been a high degree of activity in the area of water-insoluble particulate hydrocolloid absorbent compositions of matter and products using the same, such as absorbent dressings, diapers, catamenial tampons, and the like for absorbing aqueous fluids such as water, urine and other aqueous body exudates. Such substantially water-insoluble compounds maintain their particular character as they imbibe and absorb many times their weight of surrounding liquid, and in doing so, swell. The compounds are capable of absoring at least 15 times their weight of fluid. In doing so, each individual absorbent particle swells or enlarges several hundred percent times its individual size without destruction of its initial particulate integrity. Each particle maintains the appoximate shape and geometry it had before contact with liquid, but the dimensions are greatly enlarged to provide for the binding of the liquid absorbed therein. As the water-insoluble compound accepts liquid, it substantially immobilizes the liquid therein, and the resulting particulate liquidswollen structure is gelatinous. The mass of swollen particulate water-insoluble particles defines an aciniform structure since each individual absorbent particle is a greatly enlarged particle, having become liquid-swollen or grape-like or acinus in form due to the liquid it has absorbed. The individual swollen particles are tacky and hence form a clustered mass of liquid-swollen particles. The particles remain in an acinus form state despite the presence of liquid in excess of their ability to absorb. The liquid-swollen particles bind their absorbed water tightly, but upon drying, they become dehydrated and return more or less to their original size. At this time they can operate more or less as before to absorb and bind liquids.
These water absorbent compositions of matter, are, however, subject to the problem variously termed in the art as lumping, poor "wet-out" or poor water dispersibility. Thus, upon exposure to the aqueous fluid to be absorbed, the absorbent compositions exhibit poor dispersibility in the aqueous medium and, at least initially, form visible clumps consisting of fluid-swollen material on the surface and relatively dry material on the inside. Expecially when used in the form of relatively fine powders, the exposed surface of the absorbent composition forms a gel-like structure which inhibits the passage of the aqueous fluid therethrough. Accordingly, the absorbent capacity of the compositions is at least initially reflective of only the absorbent capacity of the surface, and not the absorbent capacity of the interior as well. A slow rate of exposure of the absorbent composition to the aqueous medium to be absorbed, the use of relatively large granules of the absorbent composition, and agitation of the aqueous medium during exposure tend to reduce the occurrence of lumping, while agitation of the aqueous medium after exposure and the passage of time tend to dissolve lumping once it has occurred. Nonetheless, in many instances the specific application in which the absorbent composition is used precludes one or more of the aforementioned expediencies, and the need remains for an absorbent composition having improved aqueous dispersibility (i.e., better wet-out and less lumping).
U.S. Pat. No. 4,043,952, assigned to the assignee herein, teaches a method for improving the water-dispersibility of such water-absorbent (water swellable) compositions of matter by surface treating with polyvalent metal cations to ionically complex the exposed outer surface of the absorbent composition.
Although the method disclosed in the above-mentioned patent provides a dramatic improvement in the water-dispersibility or "wet-out" properties, the need for further improvement still exists. This need is particularly apparent when the water-absorbent composition is recovered and utilized in its dry form. Thus, it has been postulated that the drying operation itself may be responsible for a decrease in the wet-out and consequent water-absorption properties. It has further been observed that conventional tray drying (either with exposure to air or oven temperatures) results in a finely divided or pulverized product in contrast to the more desired granular form.
It is therefore an object of the present invention to provide a method for improving the "wet-out" properties of absorbent compositions of matter.
It is a further object to provide a method for improving "wet-out" properties while retaining the compositions in granular agglomerated form.
These and other objects will be apparent from the description which follows.